Except in terms of duration, the increment occurs at the completion of the time period. If you had a box of years, you would count them 1-10. When you count years as a matter of duration, Year 1 actually signifies the completion of Year 0. It works this way for years of service at your job, calculations for retirement benefits, and a bunch of other things.
As for decades, it depends on who you ask, and in the end it really doesn't matter anyway.